Psychology of Learning

Module 05: Classical Conditioning 2

Part 1: Challenges to Traditional Views of Classical Conditioning

Looking Back

In Module 04, we explored the fundamentals of classical conditioning—Pavlov’s discovery of how neutral stimuli acquire the power to elicit responses through association with unconditioned stimuli, the basic process (CS paired with US produces CR), factors affecting acquisition, extinction, generalization and discrimination, higher-order conditioning, sensory preconditioning, sign tracking, conditioned emotional responses, and conditioned inhibition. Throughout Module 04, we treated classical conditioning as a relatively uniform process—the same principles seemed to apply across different stimuli, species, and situations. But is this really true? Are all CSs equally conditionable with all USs? Does classical conditioning always require multiple pairings and short CS-US intervals? As researchers investigated conditioning more deeply, they discovered important exceptions that challenged traditional views and revealed biological constraints on learning.

A Puzzling Phenomenon: The Ice Cream Story

A family once used a home ice cream maker to create a batch of ice cream from fresh peaches. The ice cream was delicious and everyone enjoyed it. However, several hours later, all family members experienced stomach flu—nausea, vomiting, the works. The illness had nothing to do with the ice cream; it was simply unfortunate timing. But from that day forward, no family member could eat peach ice cream without feeling queasy. The sight and smell of peach ice cream elicited nausea, even though everyone knew rationally that the ice cream hadn’t caused the illness (Bernstein & Webster, 1980).

You, a friend, or a family member likely can relate a similar story. Perhaps you ate pizza before getting stomach flu and now can’t stomach pizza. Or you got sick after drinking tequila and now the smell makes you nauseous. These experiences illustrate taste aversion learning—a form of classical conditioning that violates many principles established in Module 04. Understanding taste aversions reveals that learning isn’t a general-purpose mechanism equally applicable to all situations. Rather, organisms come biologically prepared to learn certain associations more readily than others.

Taste Aversion Learning: A Special Case

Taste aversion learning is a special case of classical conditioning in which consumption of a novel flavor (CS) followed by illness (US) results in avoidance of that flavor (CR). Unlike typical conditioning, taste aversions can develop after a single pairing and with delays of many hours between CS and US (Garcia & Koelling, 1966).

In normal classical conditioning, the CS must precede the US very shortly—typically within 0.5 seconds—for conditioning to develop. After all, cause and effect need to be closely paired in time. But in taste aversions, illness (US) can follow consumption (CS) hours later—up to 24 hours—and still produce strong conditioning. This dramatic exception to the temporal contiguity principle suggests something special about taste aversion learning (Garcia, Ervin, & Koelling, 1966).

Taste aversions seem to demonstrate that organisms are biologically predisposed to learn associations important to survival. If you eat something and get sick, your survival depends on avoiding that food in the future—even if sickness occurs hours after eating. Evolutionary pressures shaped nervous systems to make this particular type of association readily, despite violating typical conditioning principles.

The Coyote & Sheep Story

John Garcia, who pioneered taste aversion research, conducted a fascinating field study. If you feed coyotes sheep meat tainted with poison, making them sick, the coyotes subsequently avoid eating sheep. More remarkably, if caged together with sheep afterward, coyotes cower in fear of the sheep! Through taste aversion conditioning, coyotes learned that sheep are dangerous—a complete reversal of the natural predator-prey relationship. This demonstrates the power and ecological significance of taste aversion learning (Garcia & Gustavson, 1997).

Ranchers have used this technique, called conditioned taste aversion, to protect livestock from predators without killing them. Leaving treated meat for predators creates aversions that reduce livestock predation while preserving predator populations—an elegant solution based on understanding how learning mechanisms evolved to serve survival needs.

Contemporary Applications of Conditioned Taste Aversion

Recent research has systematically examined the use of conditioned taste aversion as a wildlife management tool. Snijders and colleagues (2021) reviewed decades of field applications across various human-wildlife conflict situations, including livestock depredation, crop raiding, and egg predation. Their analysis identified key factors affecting success: stimulus novelty (aversions form more readily to unfamiliar foods), stimulus salience (distinctive tastes produce stronger aversions), generalization (whether aversion extends to untreated versions of the food), and extinction prevention (maintaining the aversion over time). While laboratory studies consistently demonstrate the power of taste aversion learning, field applications have shown mixed results, highlighting the importance of applying learning theory principles when designing interventions. This work demonstrates how basic research on biological constraints in learning continues to inform practical conservation and management strategies.

Bait Shyness in Rats

If you don’t kill rats the first time with poison, they become “bait shy“—they avoid that poison bait in the future. This creates enormous problems for pest control. Rats that survive poisoning develop taste aversions to the poison’s flavor, making them nearly impossible to poison again with that particular bait. This practical problem led researchers to investigate taste aversions systematically, ultimately revealing fundamental principles about biological constraints on learning (Barnett, 1963).

Basic Procedure for Establishing Taste Aversions

The basic procedure is straightforward. Consumption of a novel flavor (typically saccharin-flavored water) serves as the CS; novelty is crucial—taste aversions establish more strongly to unfamiliar flavors. A food that might have become a signal for consumption (appetitive response) becomes a signal for aversion. Illness induction (through injection, radiation, or toxin) serves as the US, eliciting nausea and vomiting (UR). The result is avoidance of the previously novel flavor (CR); animals refuse to consume it or show greatly reduced consumption.

Testing for Taste Aversions

Researchers use two main methods to test for taste aversions. The one-bottle test compares consumption of the novel flavor between taste aversion conditioned animals and a control group that hasn’t received conditioning; conditioned animals drink significantly less, demonstrating the aversion (Garcia & Koelling, 1966). The two-bottle test is useful for detecting weak aversions; it asks thirsty animals to choose between a familiar flavor (like water) and the novel flavor (like saccharin). Animals that received taste aversion conditioning drink significantly less of the novel flavor than control animals. The choice situation makes even weak aversions detectable (Garcia & Koelling, 1966).

Challenges to the Traditional View of Classical Conditioning

Taste aversion learning challenges traditional conditioning principles in four major ways.

1. Single Versus Multiple CS-US Pairings

Traditional Principle: Most classical conditioning requires multiple pairings for acquisition. Pavlov’s dogs needed many tone-food pairings before salivating to the tone alone. Each pairing gradually strengthens the association.

Taste Aversion Exception: Taste aversions often develop after just one pairing. Eat novel food, get sick once, avoid that food forever. Single-trial learning is the norm, not the exception, for taste aversions. This makes adaptive sense—organisms can’t afford to “practice” eating poisonous foods multiple times to learn they’re dangerous (Garcia & Koelling, 1966).

2. Optimum CS-US Interval

Traditional Principle: The optimal CS-US interval is approximately 0.5 seconds. Shorter or longer intervals produce weaker conditioning. Beyond a few seconds, conditioning practically disappears. This reflects the need for temporal contiguity—cause and effect must be closely paired.

Taste Aversion Exception: Taste aversions can develop with CS-US intervals as long as 24 hours! Consume novel food at breakfast, get sick at midnight, still develop aversion. This violates the temporal contiguity principle dramatically. The interval can be extended greatly if distracting information is eliminated—animals remember what they ate (Garcia, Ervin, & Koelling, 1966).

3. Long-Lasting Nature

Traditional Principle: Many conditioned responses extinguish relatively quickly if the CS is presented without the US. Extinction occurs within experimental sessions.

Taste Aversion Exception: Taste aversions can be extremely long-lasting and resistant to extinction in both animals and humans. People report aversions lasting decades. Even after numerous exposures to the food without illness, aversions persist. This resistance to extinction also serves survival—better safe than sorry when it comes to potentially toxic foods (Rozin & Kalat, 1971).

4. Cue-to-Consequence/Preparedness Effects

Traditional Principle: Researchers initially believed any CS could be equally well conditioned with any US. Stimuli should be interchangeable—lights, tones, tastes, smells should all condition equally well to any US.

Taste Aversion Exception: Certain stimuli associate more readily with certain USs. This selectivity in what associates with what violates the equipotentialityassumption of traditional conditioning.

Garcia & Koelling’s Landmark Experiment

The most famous demonstration comes from Garcia & Koelling (1966). They trained rats using a compound CS consisting of three elements presented simultaneously: flashing light + clicking sound + saccharin flavor. This “bright-noisy-tasty water” was followed by either electric shock (US) or illness (US).

When tested later with individual elements, the Shock Group showed that flashing light + clicking sound (audiovisual cues) elicited strong CR (fear), while saccharin flavor elicited weak CR. The rats associated external cues (light and sound) with external consequences (shock). The Illness Group showed that saccharin flavor elicited strong CR (aversion), while flashing light + clicking sound elicited weak CR. The rats associated internal cues (flavor) with internal consequences (sickness).

This selective association pattern—dubbed “preparedness“—suggests rats are biologically prepared to associate internal cues (flavor) with internal consequences (sickness) or external cues (sights and sounds) with external consequences (shock). Not all CS-US combinations are equally learnable (Garcia & Koelling, 1966).

Preparedness is Species-Specific: The Quail Studies

Preparedness isn’t universal across species—it reflects each species’ unique evolutionary history and sensory capabilities. Wilcoxon, Dragoin, & Kral (1971) demonstrated this beautifully using quail.

In Experiment 1, quail were presented with blue-colored, sour-tasting water (CS), then induced illness (US). When tested, half received sour-tasting (but not blue-colored) water; the other half received blue-colored (but not sour-tasting) water. Result: The sour-tasting water was readily consumed, whereas quail refused to drink blue-colored water. Quail conditioned to color but not to taste!

Why? Quail are visual foragers with excellent color vision. They rely heavily on visual cues to identify food. In contrast, rats have poorly developed vision but excellent gustatory (taste) and olfactory (smell) systems. It makes adaptive sense for quail to be innately prepared to associate visual stimuli with food-related consequences and for rats to be prepared to associate taste and smell stimuli with these consequences (Wilcoxon et al., 1971).

Preparedness: Biological Constraints on Learning

Preparedness refers to the innate potential to associate certain stimuli more easily and quickly with certain USs. Organisms are “prepared” by evolution to form some associations readily, while being “unprepared” or even “contraprepared” to form others (Seligman, 1970).

The gustatory and olfactory systems of rats are highly developed, whereas their visual system is not. The converse is true for quail, who have excellent vision but less developed taste and smell systems. Each species’ sensory specializations reflect its ecological niche and feeding strategies. Evolution shaped learning mechanisms to capitalize on species-specific sensory strengths.

Preparedness extends beyond taste aversions. When it comes to phobias (learned through classical conditioning), humans easily develop phobias to snakes, spiders, heights, and enclosed spaces—evolutionarily relevant threats. But it’s difficult to condition phobias to flowers, cars, or electrical outlets—modern dangers without evolutionary history. People readily develop snake phobias after seeing pictures of snakes paired with shock, but not flower phobias under identical conditions (Öhman & Mineka, 2001).

Summary: Is Taste Aversion Learning Unique?

With all these unusual features, is taste aversion learning a completely different form of learning? Probably not. Selective associations (preparedness) have been demonstrated in other classical conditioning studies beyond taste aversions; preparedness is a general principle, not unique to taste learning. One-trial learning can sometimes occur in other situations if the CS is really obvious and the US is extremely strong; single-trial learning isn’t exclusively a taste aversion phenomenon. The CS-US interval can be greatly extended in other situations if distracting information is eliminated and memory demands are reduced.

So taste aversion learning isn’t a fundamentally different type of learning—it’s classical conditioning operating under biological constraints. It reveals that learning mechanisms evolved to solve specific adaptive problems, not as general-purpose association machines. Understanding these biological constraints provides deeper insight into how and why learning works.

Looking Forward

We’ve seen how taste aversion learning challenged traditional views of classical conditioning, revealing biological constraints and preparedness effects. Not all stimuli are equally associable with all outcomes—evolution shaped learning mechanisms to capitalize on species-specific sensory capabilities and ecological needs. In Part 2, we’ll explore additional phenomena that challenge simple views of conditioning: cue competition effects like blocking and overshadowing, cue facilitation effects like potentiation, and the puzzle of drug tolerance as a conditioned response. We’ll also examine extinction effects including facilitated reacquisition and renewal, phenomena showing that extinction doesn’t simply erase original learning but creates new, context-dependent learning that coexists with original associations.